Add Google Guetzli optimizer

[chinshr]

Do you think it is a good idea to include Google Guetzli?

If you think it's a good idea, I can spend some time adding it here. Any info on how to integrate is appreciated.

[toy]

Probably. It seems to be of interest ;)
I did not yet fully analyse guetzli (efficiency, "compatibility" with other tools), though started checking how easy to add here. And toy/image_optim#145 is already waiting.